This invention relates generally to the placing of objects into alignment and more specifically to the precise rotational and translational positioning of objects with respect to one another in which the accuracy of the alignment is verified.
In the manufacture of integrated circuits it is necessary to expose with a series of photomasks successive photoresist layers which are used to mask portions of the substrate during processing. Each exposure must be correctly aligned with the existing patterns on the substrate or the yield of finished product will be impaired. This alignment has been done manually in the past by operators who observe the mask and wafer patterns through a microscope or other visual means. In some cases special alignment targets are provided which the operator superimposes. Correct alignments have been found to be subject to human error and fatigue. In attempts to increase throughput and obtain more consistent alignments, automatic alignment systems have been developed in which electro-optical means are employed in order to sense when the wafer and mask patterns, or alignment targets, are in correct alignment. However, because of the varied quality and nature of the patterns being aligned, the automated systems have also been found to be not completely satisfactory in achieving consistent alignments.
In both manual and automated systems it is also difficult to determine whether the alignment has been correctly performed until after the processing step has been completed. Where errors in alignment occur in the later processing steps, the resultant yield losses can be costly.
The subject invention provides for the aligning of objects, such as a mask and resist coated semiconductor wafer, in which positive control over the accuracy of the alignment is exercised.